1. Technical Field
The present invention relates to conveyance devices for sheet media stored in a roll, and relates more particularly to media conveyance devices that can quickly select the appropriate drive mode for a roll rotating unit to convey the sheet medium in reverse.
2. Related Art
Receipt printers and other devices that use sheet media (such as paper) stored in a roll have a device for conveying the media to a processing position. The conveyance device usually has a drive roller that feeds the media from the roll to the conveyance path, and a roll rotating device that rewinds the conveyed media. The media is conveyed forward and reverse by driving these rollers.
The amount of media held in the roll, that is, the diameter of the roll, changes as the media is conveyed and consumed, and the load on the conveyance operation therefore changes. The roll diameter must therefore be known and reflected in the conveyance operation in order to accurately control media conveyance. Accurately determining the roll diameter is particularly important when the media is rewound by driving the roll rotating device because the conveyance speed is determined by the roll diameter.
Methods that determine the initial roll diameter by performing a special operation to measure the roll diameter during the device initialization process and then estimate the roll diameter thereafter based on media conveyance distance and media thickness (paper thickness) information are known from the literature.
Japanese Unexamined Patent Appl. Pub. JP-A-2008-254826 is directed to a method of detecting how much roll paper remains while rewinding the roll paper in a roll paper recording device. Japanese Unexamined Patent Appl. Pub. JP-A-H 10-147463 is directed to using the roll diameter for conveyance control in a compact web winding device that constantly maintains optimum tension and prevents media slack and biasing.
A problem with the roll diameter estimation method of the related art is that extra time is required for the special operation described above. Moreover, the estimated roll diameter is not very accurate due to error in the measured conveyance distance and variation in the media thickness.
The roll diameter is acquired in the process of measuring the remaining amount of roll paper in the method described in JP-A-2008-254826, but this value is acquired during the reverse conveyance operation. Because using the correct roll diameter is particularly important for controlling conveyance in reverse as described above, the actual current roll diameter is preferably acquired before reverse conveyance starts. However, the value from the previous reverse conveyance operation must be used if the roll diameter is acquired during reverse conveyance, and because the media is typically conveyed forward after being reversed, the actual roll diameter of the previous reverse conveyance operation cannot be accurately used for the next reverse conveyance operation.
Conveyance devices such as described above also typically have a number of different drive modes in which the drive units operate at different (rotational) speeds. The desirable mode is preferably selected appropriately according to the conveyance requirements.
When selecting and setting the drive mode, the diameter of the drive rollers is constant and the appropriate drive mode can be determined relatively easily. However, because the conveyance speed of the roll rotating device described above varies according to the constantly changing diameter of the media roll and the relationship between the different drive units must also be considered, determining the appropriate drive mode becomes a complicated process.